Mobile cell telephones, two-way radios, and other wireless client devices (WCDs) are experiencing tremendous growth in the U.S. and around the globe, giving users the ability to place a call, including an emergency call, from almost anywhere at any time. One important feature that is being included in most modern WCDs is an ability to determine and report a current location of the WCD.
Various methods to locate a WCD, at least approximately, may be used. One known approach to determine the location of a WCD is ground-based triangulation. In a ground-based triangulation system, the WCD's location is identified through a ranging technique between a plurality of base stations and a transponder at the WCD.
Another known technique is based on a determined time difference of arrival (TDOA) in which a data burst is received simultaneously at three base station sites. From the time difference of arrival of the data burst from the WCD at each of the base station sites, the approximate location of the WCD can be determined.
In a third known technique, a Global Positioning System (GPS) of satellites and a corresponding receiver in the WCD may be used. GPS is made up of more than two dozen earth-orbiting GPS satellites. Each satellite contains a computer, an atomic clock, and a radio. Each satellite continually broadcasts its changing position and time. These precise timing and position signals broadcast in radio frequency, allow the GPS receiver in the WCD to accurately determine a location of the GPS receiver (longitude, latitude, and/or altitude) anywhere on Earth that has a direct or indirect view of the orbiting satellites. The GPS satellites are positioned in orbit in a manner such that from any given point on Earth, at least four GPS satellites are visible above the horizon at any one time. The GPS receiver in the WCD also contains its own receiver and a computer that calculates its position using a process called trilateration, which is similar to triangulation. The GPS receiver calculates time signals from at least three GPS satellites to measure its distance from each satellite and to calculate a result. The calculation result is provided in the form of a geographic position (longitude and latitude). The location accuracy may be anywhere from 1 to 100 meters depending on the type of equipment used and the number of satellites visible. The GPS is owned and operated by the U.S. Department of Defense, but is available for general use around the world. The European Space Agency and the European Union are also building out a separate global navigation satellite system called Galileo, among other additional projects being pursued by other countries.
In one example, the GPS data payload, used to provide other devices with the determined position of the WCD, may use a 32-bit long absolute latitude coordinate and a 32-bit long absolute longitude coordinate. Given the frequency of location updates, and the number of WCDs transmitting location updates, the transfer of 64-bits of location data, in addition to overhead required to transfer the 64-bits of raw location data, generates significant traffic over a wireless network. For example, using the Motorola Solutions™ MOTOTRBO Location Request and Response Protocol (LRRP), in which LRRP control and data messages are sent via a wireless radio network within UDP/IP packets that are transported over the common air interface (CAI), 6-7 LRRP transmission bursts may be required to transfer the 64-bits of raw location data, including overhead, taking up more than 300 ms of transmit time per update.
Clearly, as more and more devices implement GPS receivers and location reporting, and as the frequency of location updates increases, GPS payload data is becoming a predominant bandwidth consumer over wireless area networks.
What is needed is a method and system for more efficiently transmitting GPS payload data, so that useful location-based services can continue to be provided and locations of WCD's provided to those agencies that rely on them, while decreasing the costs and network utilization rates inherent in providing such services.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.